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In many cases the mold and bacterial growth on the evaporator coil or adjacent ducting can better be solved by increasing house airtightness (to reduce humid outside air infiltration) or installing dehumidification to drop the overall amount of moisture in the house, because many times it is just because there is too much moisture in the household air that is circulating around but not being removed.

In other cases, if overall moisture in the house is not excessive, just increasing the "rundown time" for the fan - how long it runs after the A/C shuts off - can solve the issue by drying off the evaporator by running air past it after it shuts off.

In some other cases, the problem is really from icing of the evaporator creating excessive moisture on it - which usually indicates an air conditioner problem - generally low refrigerant level, especially if this is a new occurrence, not a long-term continuing problem.

What you need is to find an old-school Heating and A/C tech who learned how to diagnose a system's problems, not just replace parts till it runs or put in a new system as the cure for a problem. Someone who can figure out (using airflow and humidity and temperature measurements) whether the coil is working right, if the airflow is proper for the cooling capacity of the coil (i.e. is right amount of energy being taken out of the air passing through the coil or is it over or under-cooling), is incoming makeup or infiltration air too humid, even if the air handler itself is in a part of the house which is too damp (common with crawlspace and unheated basement installations), etc.